The development of charge images of images to be printed that are applied on a charge image carrier (for example a photoconductor drum or a photoconductor belt) by a character generator (for example an LED character generator) in an electrophotographic printing or copying device is known, for example according to the toner jump principle (see for example U.S. Pat. No. 4,868,600). In this principle, in the development region a toner cloud of toner particles is generated in the intervening space between developer roller (jump roller) and charge image carrier via application of an alternating voltage and/or a direct voltage (bias voltage), from which toner cloud toner particles cross over onto the charge image carrier, corresponding to the charge images, and ink the charge image carrier.
The charge images on the charge image carrier can be generated by an LED character generator. This can recharge individual output pixels or PELs (printed elements) via exposure, which individual output pixels or PELs are in a print raster made up of addressable output pixels on the charge image carrier depending on the character to be printed. These PELs are then developed into printed dots via the developer station. A printed dot is thus the dot that is physically printed at the location of the PEL; it is normally larger in area than the corresponding PEL. The printed pattern can be divided up into raster cells; one raster cell is thereby a two-dimensional matrix of PELs.
In operation it is necessary to establish the character width of a character. What is thereby to be understood by character width is how wide or how fat a printing device outputs a predetermined character. The appearance of the print image and the toner consumption can be affected by varying the character width.
The character width can be measured with the aid of an optical reflex sensor that measures the (infrared) light cast back by the surface of the charge image carriers. Integration thereby takes place over a surface of a few square millimeters in size (a few thousand printed dots). In principle the character width can be determined by measuring a print raster on the charge image carrier. It applies that: strong reflection=small printed dots=narrow characters, weak reflection=large points=wide characters. Since the reflection is different for different toner colors, the character width can only be measured depending on the toner color via direct measurement of a print raster with a reflection sensor. Given different colors this is difficult, in particular given mixed colors. The contamination of the sensor and the contamination or discoloration of the charge image carrier can additionally adulterate the measurement.
The measurement can also take place in that a toner mark is generated on the charge image carrier, the toner quantity of which toner mark is determined via capacitive toner quantity measurement. The toner quantity changes depending on the printed dot diameter or the line width of the toner mark.
Manipulated variables for the printed dot and line variation in characters that are to be printed are, for example, the bias voltage at the jump roller, the charging/dischargind potential of the charge image carrier, and properties of the developer mixture.
The measurement of a toner mark with an optical reflex sensor is known from U.S. Pat. No. 7,016,620 B2, for example; the measurement of a toner mark with a capacitive toner quantity sensor is known from U.S. Pat. No. 7,260,334 B2.